Adhesive composition blends of curable epoxy resins with various thermoplastic polymers are known. For example, adhesive compositions comprising epoxy and polyester thermoplastic resins have been used as hot melt adhesives and in the manufacture of structural bonding and scaling tapes.
However, adhesive compositions consisting of epoxy resins and thermoplastic polyesters have several deficiencies. For example, they can be prone to excessive flow prior to or during cure, unless flow control additives are incorporated. This excessive flow may result in a messy adhesive bond line that must be smoothed out in a subsequent step in cases where appearance is important (for example, exterior automotive applications such as trim adhesion, body panel and door skin replacement). Additionally, excessive flow can form gaps in the bond line which may lead to bond failure due to moisture penetration or corrosion. Also, those epoxy-polyester adhesive compositions that have been formulated not to undergo excessive flow typically lack sufficient tack to be used in the form of a thermosettable pressure-sensitive tape without incorporation of additional ingredients or a separate adhesive layer. Further, in order to obtain tack and good adhesion to painted metal substrates it is often necessary to increase the epoxy content of the adhesive composition above that level which otherwise achieves optimal properties.
Thermally curable compositions of epoxy resins having ethylene-vinyl acetate (also called "EVA" herein) copolymer particles dispersed therein have also been disclosed as toughened epoxy resins containing a dispersed phase of EVA polymerized in situ.
Epoxy material containing compositions having thermal curatives or hardeners such as those described above are difficult to melt process into articles (for example, tapes, films, or rods) without causing premature curing of the composition. Use of high temperature curatives that do not cause curing during such processing steps requires a high cure temperature during formation of the adhesive bond, which can cause excessive flow of the adhesive during cure thereby resulting in an aesthetically unpleasant or structurally defective bond or thermal damage on the case of thermally sensitive substrates.